It started when he was 12, during a flight from Charlotte, N.C., to Savannah, Ga., in a 12-seater plane. The engine started smoking, and passengers began to panic. They landed safely, but the memory of that experience — the helpless feeling and his racing heart— was seared in his mind.

Overnight, Adam morphed into a fearful flyer. He would take the bus or train, anything to avoid planes. Over the years, his fears multiplied. He started feeling claustrophobic in subways, then petrified while driving on bridges — hyperventilating and anxious that his heart was racing so fast he might die of a heart attack.

“Everything I did from the moment I got up was to plan to avoid things . . . planning my routes around tunnels and bridges, it was so exhausting,” said Adam, a 30-year-old North Shore marketing consultant.

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Adam is hardly alone. An estimated 12 percent of US adults have struggled at some point in their lives with a specific phobia, such as fear of flying, insects, or getting shots, according to the National Institute of Mental Health.

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Specialists say that most phobias, which are an exaggerated, usually inexplicable and illogical fear of a specific object, situation, or activity, are relatively easy to treat. Yet people often don’t seek help, usually because they are embarrassed or don’t realize treatment can be effective.

The results of two recent studies, one involving the use of an anti-tuberculosis medication, and another involving sleep, show promise.

Both build on the most common phobia treatment, which uses an approach called exposure therapy, in which patients are gradually exposed to the object or situation most feared, such as spiders, or driving over bridges, while learning how to relax their breathing and control their racing, fearful thoughts.

Exposure therapy is the type of treatment that ultimately worked for Adam, who asked that his last name not be used because he is worried that disclosure of his phobia may hurt his business. He sought help at Boston University’s Center for Anxiety and Related Disorders.

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One of the center’s researchers, psychology professor Stefan Hofmann, has been studying the use of d-cycloserine, a government-approved anti-tuberculosis treatment, for use in phobia therapy. In a recent study, Hofmann and colleagues found that patients with a fear of heights who received the medication immediately after an exposure therapy session had significantly greater improvement in symptoms compared with patients who received a placebo. The medication worked in those whose fear level was low after the session.

But there was a catch: They found the medication had the opposite effect and enhanced fear in patients whose fear level was still relatively high after a therapy session.

“If exposure [therapy] is not successful, the drug enhances the unsuccessful part,” Hofmann said. “If exposure therapy is successful, the drug makes it more successful.”

One other complication: d-cycloserine is most effective when given before therapy, Hofmann said, making it hard for therapists to know which patients should receive it. He’s working on a study now that, he hopes, will help therapists sort that out. The drug’s antibiotic action in fighting TB is different from the way it apparently works to enhance learning in the brain of phobia patients.

Chicago researcher Katherina Hauner has been tapping into patients’ sleep for potential phobia treatments. In a study involving 15 people, Hauner found that a fear memory could be reduced by exposing people to the memory over and over while they slept — a time when, it’s believed, memories are strengthened.

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To create a fear memory, the 15 participants received mild electric shocks while seeing two different faces. They were also simultaneously exposed to a specific scent, such as mint, lemon, or pine, as they viewed each face, so the face and scent were associated with fear.

Daniel Mears//Detroit News/AP

Regan Skinner, 10, screamed during a haunted car wash in Michigan.

The 15 subjects then slept for about an hour, while one of the scents was released, repeatedly reactivating the fear memory associated with one of the faces. When they woke up, their fear reactions, measured by sweat on their skin and brain scans, were lower when they saw the face linked to the smell they had been exposed to during sleep, compared to their reactions when seeing the other face.

The study, published last month in the journal Nature Neuroscience, had no follow-up and was too small to indicate whether the effects were lasting, said Hauner, who did the research while at Northwestern University Feinberg School of Medicine.

Hauner does not envision sleep therapy replacing more traditional approaches, but perhaps it could be used as an adjunct, if future studies indicate it’s effective.

“If you can kind of [home] in on this period when memories are being replayed, and modify it by giving the brain new information, then perhaps you can make changes that would enhance what was learned during the day, or modify it a bit,” she said.

Women are far more likely to struggle with phobias, for reasons that aren’t clear, and many common phobias — fear of small animals, clowns, or heights — typically start in childhood, said Dr. David Barlow, a BU psychology and psychiatry professor, and founder of the school’s anxiety center.

Social anxiety and situational phobias, such as fear of flying and claustrophobia, typically develop in later teenage years or when people are in their 20s, Barlow said.

Several factors are believed to influence how and why people develop phobias.

“There is some genetic influence, an inherited tendency to be uptight to begin with, but that alone isn’t responsible,” Barlow said.

About half of people with phobias had a particular experience or encounter, with a scary dog, for instance, that appears to have sparked the fear. But many people cannot pinpoint a specific event. They may have flown for years, then unexpectedly had a panic attack — a sudden surge of adrenaline, racing heart, rapid breathing — while on a plane for no apparent reason, Barlow said.

For some, the panic attack may be an isolated event. But others attach that “fear response” to whatever they might be doing at the time, such as flying or driving over a bridge, and a phobia is born, Barlow said.

Some phobias can be vanquished through a single exposure therapy session, while others may take several weeks or months of treatment.

Hauner, the Chicago researcher, helped 12 adults tame their lifelong fear of spiders in one, three-hour session that was so successful, they were able to hold a tarantula in their hands six months after treatment, according to her 2012 study published in the Proceedings of the National Academy of Sciences.

The session involved progressive steps, from looking at photos to slowly approaching an enclosed tarantula and eventually touching it, with each step first demonstrated by the therapist. During the session, participants were taught about tarantulas, learning that they are predictable and far less threatening than many imagined, Hauner said.

Scans taken immediately after treatment showed less activity in an area of the brain associated with fear when the participants viewed photos of spiders, compared with those taken before treatment, a change that persisted six months later.

Treatment for Adam, the North Shore marketing consultant, took considerably longer. Four years ago, he started twice-weekly sessions to deal with his fear of bridges, subways, and other enclosed spaces. A therapist would accompany Adam into crowded subways and talk him through his irrational thoughts, explaining how unlikely it would be for an otherwise healthy 26-year-old to die from his fear.

After three months of sessions that tapered to every other week, Adam conquered all but his most persistent phobia — flying. Last year, he returned to BU’s anxiety center, and in a one-day session, he copiloted a three-seater plane out of Hanscom Field in Bedford, with his therapist again at his side.

He was petrified, but proud of the accomplishment, though he has yet to climb aboard a commercial flight.

“I don’t think I will ever be totally OK sitting 30,000 feet up in the air in a tube with someone else controlling my destiny,” he said. But he is ready to try.

Adam is planning a trip to Florida, on a commercial airliner, in the spring.